luxuriant growth, beautiful
萋 centers on lush, abundant growth, especially of plants, and by extension a flourishing beauty. The two given meanings are closely linked: luxuriant vegetation is visually beautiful.
萋 combines the grass radical 艹 with 妻, which likely contributes the sound. The character is used to describe lush vegetation.
The grass radical 艹 on top of 妻 (wife) suggests a wife tending a garden so well that the plants grow thick and beautiful.
For セイ, imagine the wife saying 'say' as she admires the beautiful, luxuriant growth: say -> セイ.
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